Previous work in this program has been directed toward the cerebral cortex, principally in the albino rat. Electron microscopic techniques with correlated analysis of light microscopic material, including Golgi preparations have been used. These investigations have contributed to understanding the development of the cortex and its normal adult structure, particularly specific neuronal and neuroglial elements. The present proposal builds upon and extends the previous work. One phase of the research will be a continuation of the developmental studies, aimed at revealing temporal correlations among such factors as cell migration, arrival of extrinsic afferents, the formation of pyramidal cell dendritic clusters, maturation of stellate cells, and appearance of specific classes of synapses and dendritic spines. Studies in adult cortex will proceed in three major directions. One aspect will focus on the detailed morphology of the neurons, including the stellate cell and its possible role in neuronal circuits. A second aspect will consist of an analysis of serial thin sections, to provide criteria for the identification of small profiles in electron micrographs. Such criteria will be used to identify the neuronal elements postsynaptic to degenerating axon terminals produced by lesions in the thalamus and corpus callosum. Once the above studies are well underway, a new phase of the work will be initiated, a study of the cerebral cortex of the rhesus monkey. The overall plan for this phase will be to use the findings on the rat as a guide to analysis of monkey cortex.